In order to reduce an amount of NOx emitted from an internal combustion engine mounted on a vehicle to the atmosphere, a NOx occlusion reduction type catalyst (LNT: Lean NOx Trap) has been used as a NOx lessening catalyst. This NOx occlusion reduction type catalyst occludes NOx when the exhaust gas is in a lean air-fuel ratio. Meanwhile, in a rich air-fuel ratio, this NOx occlusion reduction type catalyst releases the occluded NOx, and reduces the released NOx in a reducing atmosphere, for lessening NOx.
In order to release and reduce the NOx occluded in such a NOx occlusion reduction type catalyst for regeneration, the NOx occlusion reduction type catalyst needs to be exposed to a reducing atmosphere. For this reason, although a diesel engine normally operates in a lean state, an air-fuel ratio rich control is performed for making the exhaust gas temporarily into a rich state by changing an operating state of the internal combustion engine in order to regenerate the NOx occlusion reduction type catalyst.
Diesel particulate filters (DPF) are used to collect PMs (particulate matters) in exhaust gas. When an amount of PMs collected in such a DPF increases, the temperature of the DPF is raised to a combustion temperature of the PMs for removal of the PMs by combustion. For the removal by combustion of PMs by raising the temperature of DPF, a temperature raising control of exhaust gas is performed. In this temperature raising control of exhaust gas, an air-fuel ratio rich control is performed.
For implementation of the air-fuel ratio rich control, an unburned fuel supply control is performed in many cases. The supply control of unburned fuel employs a post-injection control for performing post-injection after main injection in an in-cylinder fuel injection control, and an in-exhaust-pipe direct injection control for injecting fuel directly into the exhaust pipe from a fuel (reducing agent) injection apparatus provided to the exhaust pipe.
For example, Japanese patent application Kokai publication No. 2007-170218 discloses an exhaust gas purifying system for an internal combustion engine including, for an NOx reduction control with a NOx storage and reduction catalyst: rich purge means for temporarily shifting an air/fuel ratio of the exhaust gas emitted from an internal combustion engine into a rich range by increasing fuel combusted in a cylinder; and fuel adding means for supplying the unburned fuel to the catalyst by injecting the unburned fuel into the exhaust gas. In the exhaust gas purifying system for an internal combustion engine, the rich purge means and the fuel adding means are selectively used, and any one of in-exhaust-pipe direct injection by a fuel addition valve provided on an upstream side of the catalyst and post-injection in which injection is performed after main injection is used as the fuel adding means.
However, in a case of the post-injection, the amount of fuel injected by the post-injection is too small to make the atmosphere of the catalyst reach a reducing atmosphere. Hence, a large-amount EGR (exhaust gas recirculation) is performed simultaneously with the post-injection. When this large-amount EGR is performed under a high load state and in a case where a combustion temperature in the engine is high, a problem is caused that the concentration of oxygen, which is required for combustion of the main injection, becomes low, and combustion in the cylinder is impaired, resulting in generation of black smoke.
Meanwhile, in a case of the in-exhaust-pipe direct injection, the amount of the injected fuel is sufficient to make the atmosphere of the catalyst reach a reducing atmosphere. Hence, no large-amount EGR is performed. However, when the in-exhaust-pipe direct injection is preformed under low load state and in a case where the catalyst temperature is low, the catalyst is not activated, and hence the fuel in an unchanged state passes (slips) through the catalyst. For this reason, there is a problem that the unburned fuel is emitted to the atmosphere in an unchanged state, and white smoke is generated.
New Long-Term Exhaust Emission Regulations for automobiles place importance on reduction of such white smoke and black smoke, as well as on tightening of regulations for PMs. Accordingly, the reduction of white smoke and black smoke has been an important issue.    Patent Document 1: Japanese patent application Kokai publication No. 2007-170218